FIG. 1 shows a schematic of a synchronous buck converter 10. The converter receives power from a DC input source 15 at a voltage Vin and provides power to a load 25 at a voltage Vout<Vin. The output voltage, Vout, is presumed to be substantially constant, e.g. as a result of filter capacitance included in load 25. The converter is a switching power converter, i.e. a converter in which energy flow between the input source and the load is controlled by controlling the times at which one or more switches are turned ON and OFF during each of a series of converter operating cycles. The converter of FIG. 1 may be operated in a continuous-mode, in which a unidirectional current flows continuously in the inductor throughout each converter operating cycle, or it may be operated in a discontinuous-mode, in which the current in the inductor returns to or crosses zero at or prior to the end of each converter operating cycle.
Waveforms for the converter 10 operating in a discontinuous mode are shown in FIGS. 2A and 2B. As shown at time t1, MOSFET switch S1 is turned OFF and the current, Io, in inductor L 40 commutates into diode 31 (which may be the body diode of MOSFET synchronous switch 30). Low resistance synchronous switch MOSFET S2 30 may be turned ON at any time between t1 and t2 as a means of bypassing diode 31 and reducing energy loss. Between times t1 and t2 the voltage across switch S2 30, Vss (FIG. 2A), is substantially zero and the current in the inductor 40 is declining. At time t2 the current IO declines to zero and switch S2 30 is turned OFF. Following the turning OFF of switch S2 a resonant ringing will occur in the resonant circuit formed by inductor 40 and parasitic circuit capacitances (e.g. parasitic capacitances, not shown, associated with switches S1 and S2 and with inductor L), as illustrated in FIGS. 2A and 2B. Contributing to the ringing are the output voltage, Vout, and energy that may be stored in inductor 40 owing to switch 30 being turned OFF slightly before or after the time at which the current IO equals zero. At time t3 switch S1 is turned ON and the voltage Vss rises to a value substantially equal to Vin.
Parasitic voltage and current oscillations of the kind shown in FIGS. 2A and 2B may create losses that can account for several percent of the total energy processed during a converter operating cycle. EMI filtering may also be needed to reduce the effects of the oscillatory “noise” on the input source 15 and the load 25. Filter elements create more circuit complexity, reduce converter power density and may contribute additional losses.
One way to reduce losses and oscillatory “noise” in a discontinuous-mode power converter is described in Prager et al, Loss and Noise Reduction in Power Converters, U.S. Pat. No. RE40,072 (the '072 patent), incorporated herein in its entirety by reference. As described in the '072 patent, and shown in FIG. 3, a unidirectional switch 50, comprising a switch 51 in series with a diode 52, may be placed across the inductor 40 in buck converter 70. After switch 74 turns OFF, the voltage across freewheeling diode 75 declines as the forward current IL in inductor 40 discharges circuit parasitic capacitances. Thereafter, switch 51 may be turned ON after the voltage V1 declines below a value Vout (because diode 52 will be reverse biased and prevent conduction in the unidirectional switch 50). With switch 74 OFF, the inductor current IL declines towards zero. Owing to reverse recovery effects (bipolar diode) and/or diode capacitance (Schottky diode), diode conduction does not cease until shortly after the inductor current passes through zero, resulting in a reverse flow of current in the inductor 40, IL. With switch 51 ON, the reverse inductor current flows in the loop formed by switch 50 and inductor 40; the inductor energy is effectively “trapped” in the inductor; and parasitic oscillations are prevented. Prior to the turning ON of switch 74 at the beginning of the next operating cycle, switch 51 is turned OFF, enabling the reverse flow of current IL to charge circuit parasitic capacitances (e.g., parasitic capacitances of switch 74, diode 75 and inductor 40, not shown) such that the voltage V1 increases, thereby reducing or eliminating switching loss associated with the subsequent turning ON of switch 74.